This research focuses on auditory development in infants. Four sets of experiments are proposed on different aspects of auditory localization: (1) newborns' response to acoustic information; (2) the development of infants' perception of an auditory illusion, the precedence effect; (3) localizaation of sound in 3-dimensional space, and (4) acuity of localization in the horizontal plane. Previous work has estimated that the newborn can directionally discriminate sounds located on the right vs the left, indicated by head turns toward the sound source. This response is fragile and occurs only under certain stimulus conditions. The proposed studies will specify some critical aspects of the stimulus, including duration of sound, spectral properties, and rate of presentation. This work should lead to a better understanding of acoustic information processing in the auditory system at birth. Between birth and 4 months of age, head turning toward sound first declines, then reappears. Around 4 to 5 months infants begin to exhibit the precedence effect, an auditory illusion that concerns spatial location. Infants will be tested longitudinally to determine if these events are related, and if their development corresponds to onset of binocular rivalry. Two new procedures will be used to explore the older infant's ability to localize sound. One method consists of recording infants' reaching in total darkness to sounding objects placed within reach in a variety of positions. We will test infants' sensitivity to acoustic information that specifies a sound's location in the azimuth and the distance away from the infant's body. Does the infant have a boundary between objects within and beyond reach? While these questions have been asked in relation to infants' response to visual cues, they have not been asked in relation to audition. The second method is the application of psychophysical techniques to measurement of infants' acuity of sound localization. The infant detects a shift in sound from a center speaker to an off-centered speaker, yielding an estimate of minimal audible angle similar to that found for adults. The newborn work concentrates on delineating critical aspects of a sound's eliciting properties while the older infant work seeks to refine and exploit the new methodologies to achieve a more complete picture of auditory development in the first year of life.